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Police in Portugal confirmed they had launched an investigation after being tipped off about the extraordinary video, captured by a passer-by.
The footage, which shows a mystery brunette straddling a bare-chested man in broad daylight in a park with families playing nearby, has gone viral .
Stunned onlookers filmed the shocking scenes at the river beach of Tabuao, near the town of Paredes de Coura in the Porto region of northern Portugal.
The woman, who protected her modesty with a sarong, can be seen looking at the men filming her and smiling during the 11-minute clip.
Police confirmed they were trying to discover the identity of the couple.
A source said they had been alerted by a local who saw the footage online.
He said: “Once it had been discovered that the video that was the subject of the complaint existed, police in Paredes de Coura alerted Policia Judiciaria officers to inform them a minor appeared in a sex video which had been posted on the internet.”
It is understood a specialist PJ unit has been given the task of identifying the couple.
The woman makes no attempt to hide her identify in the X-rated footage - and at one point even talks to the girl while bouncing up and down on her partner.
Disgusted social media users called for the pair to be punished.
Elvira Nogueira said: “They’re not worthy of being parents. They should protect this youngster from intimate acts like this.”
Susana Pereira added: “What I’m seeing in this footage is perverse.”